The present invention relates to a multi-valued quadrature-amplitude-modulation (hereinafter abbreviated as QAM) signal receiver, and particularly to a receiver unit of the QAM signal which can correct demodulation distortion accompanying carrier frequency conversion of the QAM signal.
Generally, an RF (Radio-Frequency) QAM signal, which is transmitted including phase-shift modulation components, is received by a receiver unit and frequency-converted into an IF (Intermediate-Frequency) signal to be demodulated. However, it is difficult to reflect the phase-shift modulation components of the RF QAM signal exactly onto the IF signal, and the phase distortion of the IF signal derived from frequency conversion causes distortion of an I (In-phase) signal and a Q (Quadrature-phase) signal which are obtained by demodulating the IF signal, resulting in code errors.
This phase distortion depends on carrier-frequency difference between the IF signal and the RF QAM signal to be converted, and therefore, differs according to an RF channel of the QAM signal to be received.
Two types of countermeasures have been considered conventionally for preventing this phase distortion.
Countermeasures of the first type are to improve phase characteristic of the frequency converter. In an example of this type, RF QAM signals of channels having different RF carrier frequencies are once up-converted into a high-frequency signal having certain frequency higher than the RF carrier frequencies. The IF signal is obtained by down-converting the high-frequency signal. By thus processing, for example, frequency conversion into the IF signal can be performed with comparatively high fidelity in the phase characteristic. However, countermeasures of this type need large circuit sizes and high costs.
Countermeasures of the second type are to correct the phase distortion making use of an equalizer, whereof an example is disclosed in a U.S. Pat. No. 5,113,142. In this prior art, an automatic adaptive equalizer is used for correcting the phase distortion which differs depending on an RF channel to be received. However, the automatic adaptive equalizer also makes high the cost of the QAM receiver unit.